Such device is already known from U.S. Pat. No. 4,574,529. In the case of the grinding wheel provided in this patent, the end of the conical grinding surface, which is closest to the drill guide piece, verges into a surface portion of the grinding wheel, which surface is disposed vertically in relation to the center axis.
Based on the specific geometry of twist drills --the highest amount of material to be ground off lies in the periphery of the drills --the most important wear of the grinding wheel happens at the end of the grinding surface which is the closest to the drill guide piece. This wear decreases continuously along with an increasing distance in relation to the drill guide piece. This leads quite soon to the formation of a slightly rounded step in the grinding wheel at said end of the grinding surface and a corresponding convex rounding of the peripheral area of the main cutting edges of the drill and thus gradually deteriorates their cutting properties. Therefore, an adjustment arrangement is provided which uses the areas of the grinding surface correspondingly, which have been less worn by the drills, as well as the surface area of the grinding wheel which is disposed vertically to the center axis, in order to avoid such formation of steps and to preserve the rising angle of the grinding surface.
It is an object of the invention to provide a device for grinding drills in which such formation of steps in the grinding wheel does not occur even without an additional adjustment arrangement being provided. With a device according to the invention, no step can be formed in the grinding wheel at the end which is closest to the drill guide piece, since there is no grinding material outside this end, in radial direction. Thus, the outer ends of the main edges of the drill are not rounded off. The rising angle of the grinding surface decreases and consequently the angle of the drill increases. However, this is tolerable within certain limits.
Certainly devices are known --for example from U.S. Pat. No. 3,753,320 --with a conical grinding surface which forms an acute edge at the end which is closest to the drill guide piece and the following surface portion of the grinding wheel. However, the guide channels are arranged in such a way that their center axes touch the end which is the farthest away from the drill guide piece.
According to another feature of the invention, it is avoided that larger pieces of the grinding wheel could break off at the edge. According to this feature, the surface portion, which forms the edge together with the grinding surface, also has a conical shape.
By another feature of the invention, an especially efficient transport of material from the grinding surface is obtained by the fact that the grinding surface constitutes an outer cone.